1. Field of the Invention
The invention relates to a power supply device, more specifically, to a cooling control for the power supply device.
2. Description of the Related Art
In recent years, a power supply device, such as a secondary battery or an electric double-layer capacitor (condenser), has been employed as a battery for a hybrid vehicle and an electric vehicle. In such a power supply device, a plurality of power storage bodies (e.g. battery cells and capacitors) are disposed close together to make the power supply device compact. Thus, the power supply device can output high electric power.
As the power supply device can output high electric power, cooling of the power storage bodies, which generate heat at the time of charging/discharging, is an important factor to make the output from the power storage bodies constant, to extend the lifetime of the power storage bodies, and to supply constant electric power. Normally, the temperature of the entire power supply device is controlled by cooling the power storage bodies using a cooling device (e.g. cooling mechanism) provided in the power supply device.
Examples of a method of cooling the power supply device (the power storage bodies) include a gaseous cooling method and a liquid cooling method. In these cooling methods, heat transferred from the power storage bodies to a gaseous or liquid cooling medium is transferred to a casing constituting a part of the power supply device, and then is discharged from the power supply device. The gaseous cooling medium used in the gaseous cooling method is much easier to handle than the liquid cooling medium used in the liquid cooling method. However, the gaseous cooling medium has lower heat conductivity than that of the liquid cooling medium. In contrast, in the liquid cooling method, the liquid cooling medium needs to be carefully handled. For example, a sealing mechanism needs to be provided to prevent leaking of the cooling liquid from the power supply device. However, the liquid cooling medium cools the power supply device (the power storage bodies) more efficiently than the gaseous cooling medium, because the liquid cooling medium has significantly higher heat conductivity than that of the gaseous cooling medium.
The gaseous cooling medium and the liquid cooling medium are different from each other in cooling characteristics, such as heat conductivity and viscosity. However, both of the cooling media function in the same manner. Therefore, if the heat generated from the power storage bodies is efficiently discharged from the power supply device through a cooling medium, the cooling efficiency is improved. Accordingly, the temperature control, that is, cooling control for the entire power supply device is performed by controlling the flow (convection) of the cooling medium. If flowability of the cooling medium is improved, the cooling efficiency is improved as well. For example, the cooling efficiency can be improved by agitating the cooling medium using an agitator to generate a forced convection.
Further, as described in Japanese Patent Application Publication No. 2005-19134 (JP-A-2005-19134), the cooling efficiency can be improved by the forced convection generated by circulating a cooling liquid in a battery apparatus using a pump. Similar technologies are described in Japanese Patent No. 2775600 and Japanese Patent Application Publication No. 9-266016 (JP-A-9-266016).
In the power supply device in which the plurality of power storage bodies are provided, if performance varies among the power storage bodies, the lifetime of the entire power supply device is decreased. More specifically, if the temperature of the cooling medium around the power storage bodies varies depending on the portion of the cooling medium, the cooling medium has a strong cooling effect on a part of the plurality of power storage bodies, and has a weak cooling effect on another part of the plurality of power storage bodies. Thus, the deterioration rate varies among the power storage bodies (or varies among portions in a single power storage body). This decreases the lifetime of the power supply device.
Accordingly, in the technologies described in the aforementioned publications, the cooling liquid is forcibly circulated (such that the cooling liquid flows into the battery apparatus through an inlet, and discharged from the battery apparatus through an outlet). However, in the power supply device in which the plurality of power storage bodies are disposed close together, the cooling liquid is not sufficiently circulated among the power storage bodies disposed close together. Therefore, for example, the temperature of the cooling liquid in areas among the power storage bodies differs from the temperature of the cooling liquid at the outer peripheries of the power storage bodies, and thus the power storage bodies cannot be appropriately cooled.